1. Field of the Invention
The present invention relates to a surface acoustic wave filter utilizing a surface acoustic wave resonator. More particularly, the present invention relates to a surface acoustic wave filter used in a high frequency region.
2. Description of the Related Art
In recent years, intensive study has been positively conducted on a surface acoustic wave device which utilizes a surface acoustic wave, in order to apply the SAW device to a filter (hereinafter "surface acoustic wave" is abbreviated as SAW). SAW filters have been positively developed along with the recent development in mobile communications in which signals of higher frequencies are used.
There are some known methods for configuring a filter using a SAW device for a high frequency band, especially for a band of several hundreds of MHz. Typical known methods include, for example: a method for configuring a filter using a plurality of SAW resonators as described in Japanese Laid-Open Patent Publication No. 52-19044; a method for configuring a filter in which each of input and output interdigital transducers of a SAW resonator is divided into a plurality of portions (such a structure is called an "interdigitated interdigital transducer structure") as described in Japanese Laid-Open Patent Publication No. 58-154917; and a method for configuring a filter in which SAW resonators are disposed adjacently and capacitively coupled to each other as described in Japanese Laid-Open Patent Publication No. 3-222512.
SAW filters are required to have a smaller size and a much improved performance, in accordance with the miniaturization of mobile communication apparatus in recent years. In addition, SAW filters are more often used in various portions of the mobile communication apparatus. Specifically, SAW filters are used as an inter-stage filter in a transmitting circuit or a receiving circuit, an output filter of a local oscillator, and the like.
In general, filters are required to have various characteristics such as a low insertion loss in a pass band, a high stop band rejection, and a steep rise toward the pass band in a frequency characteristic. However, it is actually difficult to simultaneously satisfy all of these requirements. Accordingly, in general, some selected characteristics are significantly improved and enhanced. Depending on the resultant characteristics, the filter is used in an appropriate application.
The characteristic which is most significantly required for the SAW filter is typically the steep rise toward the pass band in the filter characteristic. However, in the SAW filter having a conventional configuration, if the stop band rejection is attempted to be increased, the steep rise in the filter characteristic toward the pass band, which is the most remarkable characteristic of the SAW filter, is typically lost.
The typical configuration of the above-mentioned conventional SAW filters will be described below.
The filter described in Japanese Laid-Open Patent Publication No. 52-19044 is configured by using a plurality of SAW resonators, and generally called "a resonator-type SAW filter". The resonator-type SAW filter generally has a frequency characteristic shown in FIG. 1, while its characteristics vary depending on a design of the SAW resonators. In this filter, the stop band rejection in the frequency region higher than the pass band can be greatly increased. However, in the frequency region lower than the pass band, the steepness of rising is degraded due to the parasitic impedance component. Thus, the filter characteristics are deteriorated.
The "interdigitated interdigital transducer structure type SAW filter" described in Japanese Laid-Open Patent Publication No. 58-154917 generally has a frequency characteristic shown in FIG. 2. In this type of filter, the ripple in the pass band is relatively large. In addition, this type of filter has another drawback in that it is difficult for its impedance to be matched with a 50.OMEGA. system.
The SAW filter having the configuration described in Japanese Laid-Open Patent Publication No. 3-222512 is generally called a "3-electrode type serially coupled SAW filter", and typically has a frequency characteristic shown in FIG. 3. This type of filter exhibits a steep rise in the frequency characteristic in the frequency region lower than the pass band, but has a drawback in that a large ripple occurs in the frequency region higher than the pass band.
In all of the above-described three types of conventional SAW filters, the input/output impedance may be disadvantageously deviated from the 50.OMEGA. system, depending on the filter characteristics to be realized. Although the filter may be able to be used even in the condition that the input/output impedance is mismatched, the filter used in such a condition cannot attain the inherent good characteristics. As a result, the filter is used with its characteristics deteriorated.
On the other hand, in order to match the impedance, it is necessary to provide an additional capacitance or inductance. However, such an additional impedance component connected results in that the filter characteristics are deviated from the desired and intended design.
As described above, the configurations of the conventional SAW filters have a disadvantage in that it is sometimes difficult to satisfactorily realize the desired filter characteristics while desirably adjusting and matching the impedance characteristic.